The present invention relates to a light quantity regulating apparatus such as a shutter and a diaphragm unit used for various types of imaging apparatus such as a video camera, a still camera, and other imaging apparatus. More particularly, the present invention relates to a shutter unit having a vane member to be incorporated in an imaging lens barrel for adjusting imaging light, or a light quantity regulating apparatus for increasing and decreasing quantity of imaging light, and an electromagnetic driving unit for opening and closing the vane member.
In general, a shutter unit and a diaphragm unit mounted on an electronic still camera and an electronic video camera have been widely known as a light quantity regulating apparatus. In the light quantity regulating apparatus, a film type vane member is fixed to a base plate (ground plate) to be incorporated in a lens barrel, so that the vane member freely opens and closes to adjust imaging light.
Such a vane member is formed of a single member or a plurality of vane members, and supported rotatably or slidably with a shaft such as a pin at a periphery of an optical axial hole in a base plate. An electromagnetic driving unit drives the vane member to open and close. The electromagnetic driving unit is formed of a rotor having a permanent magnet and an exciting coil for applying a rotational force to the rotor. Recently, a size and weight of a camera has been reduced, and it has been required to provide a quantity regulating apparatus having small power consumption, a small size and light weight.
Conventionally, in the electromagnetic driving unit, a rotor is formed such that a cylindrical permanent magnet is rotatably supported inside a coil bobbin with a coil around an outer circumference thereof. The coil is wound around an outer circumference of the coil bobbin, and a yoke is provided for magnetically shielding an outer circumference of the coil. The driving unit with such a structure tends to have a large size, especially, have a core having a large outer diameter, in which a rotary shaft is disposed in a magnet rotor.
Japanese Patent Publications (Kokai) No. 2001–298936 and No. 2002–049076 have disclosed structures in which a magnet rotor formed in a hollow cylindrical shape and an annularly wound exciting coil are arranged vertically in an axial direction. A yoke formed of a soft magnetic material is provided for guiding a magnetic field of the exciting coil around the magnet rotor to form a magnetic pole. More specifically, in the apparatuses disclosed in Japanese Patent Publications (Kokai) No. 2001–298936 and No. 2002–049076, a magnet rotor is formed in a hollow cylindrical shape; an inner yoke is disposed at a central opening of the rotor; and an outer yoke is disposed at an outer circumference of the rotor so as to surround the magnet of the rotor. A coil is provided at a position away from the magnet rotor, and the inner and outer yokes guide a magnetic force of the coil to an area around the magnet rotor, thereby generating a rotational force.
In such an electromagnetic driving unit, the hollow magnet rotor is disposed inside the outer yoke having a thin-wall cylindrical shape, and the inner yoke having a rod shape is disposed inside the magnet rotor. Accordingly, it is possible to reduce an outer size of the apparatus. At the same time, the magnet rotor receives rotational torque at an outer circumferential portion thereof near a tolerable outer diameter of the apparatus. Accordingly, it is possible to obtain a large rotational force in the small apparatus. A magnetic circuit is formed of the outer yoke, magnet rotor, and inner yoke in this order. With this structure, it is possible to minimize a gap between the outer yoke and magnet rotor and a gap between the magnet rotor and inner yoke to a minimum level for allowing rotation of the rotor. Accordingly, it is possible to reduce power consumption and increase performance of the magnetic circuit as a whole.
In the electromagnetic driving unit with such a structure, the magnet rotor is rotatably supported with the following structure. In the apparatuses disclosed in Japanese Patent Publication (Kokai) No. 2001-298936, the inner yoke disposed at the central portion of the hollow cylindrical permanent magnet is formed in a hollow cylindrical shape, and a rotary shaft of the magnet rotor passes through a central hole of the inner yoke, so that both ends of the rotary shaft are rotatably supported on the base plate and outer yoke. In the apparatuses disclosed in Japanese Patent Publication (Kokai) No. 2002-049076, the inner yoke with a column shape is disposed in the central hole of the permanent magnet with a hollow cylindrical shape, and a bearing recess is formed in an end surface at a free end of the inner yoke. Both ends of the rotary shaft provided at the central portion of the magnet rotor are supported on the recess formed in the end surface of the inner yoke and a bearing hole formed in the base plate.
In the supporting structure disclosed in Japanese Patent Publication (Kokai) No. 2001-298936, the inner yoke is disposed inside the magnet rotor, and the rotary shaft of the rotor is disposed inside the inner yoke. Accordingly, the apparatus tends to have a large diameter. Moreover, it is difficult to align the inner yoke with the cylindrical shape while the rotary shaft is formed at the center of the rotor. When the rotor rotates, a magnetic gap between the magnet and inner yoke is easy to fluctuate, thereby making the apparatus large.
In the bearing structure disclosed in Japanese Patent Publication (Kokai) No. 2002-049076, the bearing recess is formed in the end surface of the shaft of the inner yoke with a column shape. Accordingly, the inner yoke has a free end surface having a V shape cut portion, thereby reducing magnetic force formed in a loop shape between the inner yoke and outer yoke. That is, the recess lowers performance of the magnetic circuit and causes a problems in the magnetic circuit formed between the outer and inner yokes. Further, in the bearing structures disclosed in Japanese Patent Publications (Kokai) No. 2001-298936 and No. 2002-049076, when a diameter of the rotor shaft is reduced, it is difficult to machine the bearing portion and obtain good durability against friction.
In view of the problems described above, an object of the present invention is to provide an electromagnetic driving unit having a small size and a bearing recess with a simple structure, wherein when a rotary shaft of a magnet rotor is supported on an inner yoke, it is possible to maintain magnetic gaps between the magnet rotor and the inner and outer yokes, respectively, and it is also possible to maintain performance of the magnetic circuit.
Another object of the present invention is to provide a light quantity regulating apparatus having a small size by reducing a size of the electromagnetic driving unit, especially, an outer diameter thereof.
Further objects and advantages of the invention will be apparent from the following description of the invention.